In a cellular telecommunication system, a plurality of user terminals within a cell communicates with a primary station serving the cell. With the subsequent generations of cellular systems, the achievable data rate has been consistently increasing. In advanced systems, such as UMTS and LTE, multi-antenna transmission/reception techniques variously described as, MIMO, precoding or beamforming are supported for transmissions from a single primary station to a mobile terminal. Thanks to the spatial selectivity of the beamforming mode, such transmission modes have enabled an important increase of the achievable data rate and of the range of communication, while maintaining the average interference level.
In order to achieve the beamforming, a typical transmitting station having an antenna array applies a set of complex coefficients (forming a precoding matrix or precoding vector) to a signal transmitted from its respective antennas, so that, the transmission stream is spatially directed towards a receiving station. In a wireless communication system such as LTE, both the base station and terminal are typically equipped with multiple antennas. This allows different MIMO operation modes along with conventional transmission modes, like SISO transmission modes. To support the base station in determining the channel conditions in view of selecting a suitable transmission mode, the mobile terminals typically measure the downlink channels for each pair of antennas, and derive a channel state report to send to the base station.
The base station can then use this information for scheduling decisions such as:                Which terminals to transmit to;        Which frequency/time/code resources to use; and        MIMO transmission mode parameters (e.g. number of spatial streams and SU-MIMO or MU-MIMO).        
In implementations of such systems, precoding codebooks are defined. These precoding codebooks may be viewed as a way of describing a precoding matrix (or precoding vector) of the channel precoding coefficients or precoding weights in a compact way, thereby reducing the amount of required signalling for indicating the precoding. These codebooks also enable the user terminal (defined in LTE as a User Equipment or UE) to report to the network a preferred precoding for downlink transmission, in the form of an index to a codebook entry.
In this case, the preferred precoding is a set of complex coefficients to be applied to the transmit antennas of the base station (defined in LTE as an eNodeB). Similarly, precoding codebooks may also be used by the base station to signal the precoding used for a transmission to the user terminal. This enables the user terminal to derive an appropriate phase/amplitude reference from common pilot symbols for demodulation of each downlink transmission.
An effective method for capturing the channel state information, is to select the entry from a codebook of precoding which, if applied at the transmitter, would lead to the highest data rate. This information could be signaled as a PMI (Precoding Matrix Indicator). The number of spatial streams assumed, or rank indicator (RI), would typically be part of such a report.
As an example of codebook design, the LTE Release 8 codebook for 4 Tx antennas has a nested structure. This means that, for a given codebook index, the precoding vectors for given transmission rank are a subset of the vectors for the next higher rank. This feature could be helpful in using the same codebook for both SU-MIMO (with multiple layers transmitted to the same terminal) and MU-MIMO (with a limited number of layers transmitted to more than one terminal).
For LTE-A, it is intended that dynamic switching between SU-MIMO and MU-MIMO transmission modes will be supported. In this case, it is desirable that the UE feedback is appropriate for both modes. This could be achieved, by sending a PMI/RI and CQI for both modes. The main differences between PMI/CQI calculations for the two modes could be:                the maximum transmission rank assumed (e.g. limited only by the number of antennas for SU-MIMO and up to rank=2 for MU-MIMO);        the power available (e.g. full power for SU-MIMO and power divided between terminals for MU-MIMO); and        possibly different assumptions about interference.        
However, it requires a higher amount of resources. One aim of the invention is to reduce the number of bits required to signal the PMI.